Charged particle energy analyzers, also called spectrometers, are used in many scientific and technical applications in which the energy distribution of charged particles such as electrons are measured. Such uses include characterizing the composition and other properties of materials in which the electron energy needs to be measured, for example, X-ray photoelectron spectrometers and electron spectrometers, and secondary ion spectrometers. Similar spectroscopes, such as secondary ion spectrometers, have been applied to other charged particles, such as energetic ions. Many scientific experiments require accurate measurement of the energy distribution of charged particles.
The performance of a charged particle energy analyzer, of which an electron energy analyzer is but one example but the most prevalent one, is gauged by several conflicting characteristics. It needs to have a narrow resolution over a reasonably large energy band and the selected energy should be easily tuned. Its resolution needs to be stable and not require repeated calibration. The energy analyzer needs to have a high detection efficiency, which results in a high throughput of analyzed samples. Of especial importance in material characterization in which secondary electrons or ions are emitted over a wide angle from the material being probed, the energy analyzer should have a wide aperture and a wide acceptance angle to thereby increase the collection efficiency. A typical requirement of a commercial electron energy analyzer is that it be able to analyze 10 to 20% of the electrons emitted from the material and to distinguish electrons whose energies differ by as little as 0.1%.
Commercial energy analyzers should be rugged, small, easy to operate, and relatively inexpensive. If these commercial characteristics can be improved, materials analysis equipment can more readily find acceptance in production environments, such as in-line processing monitors in the semiconductor industry. Such characteristics are also important for remote operation, such as the search for life on Mars. For space applications, an energy analyzer needs to be lightweight, a characteristic also desired for other applications.
Dispersive energy analyzers rely upon electrostatic or magnetic deflection of the charged particles to select the energy of the particle to be detected. Although effective at very high resolution, dispersive energy analyzers tend to be large and have relatively small acceptance apertures, which result in a low measurement throughput. On the other hand, non-dispersive energy analyzers typically rely upon serially arranged low-pass and high-pass energy filters to allow only the particles within a selected energy band to reach the detector. A low-pass filter passes particles having energies below a cutoff energy and blocks those above. A high-pass filter passes particles having energies above another cutoff energy and blocks those below. It is understood that the cutoff energy need not represent a sharp discontinuity in the transmission factor, which may vary somewhat gradually across the cutoff energy.
Two of the present inventors disclose a compact non-dispersive energy analyzer for analyzing the energy of electrons in the range of a few electron volts (eV) to a few keV in U.S. patent application Ser. No. 10/961,631, filed Oct. 8, 2004 and published as U.S. Patent Application Publication 2005/0045832 A1, incorporated herein by reference. This energy analyzer includes an initial low-pass filter followed by a high-pass filter, both of which incorporate biased electrical grids through which the charged particles of the proper energy may pass. In particular, the low-pass filter includes a curved grid which together with a similarly curved electrode in back of it reflects the low-energy electrons into a collimated beam, which then passes through a planar high-pass grid filter. The energy overlap of the low-pass and high-pass filters determines the overall pass band of the energy analyzer, which is tuned to provide an energy spectrum.
The described energy analyzer provides superior performance. However, we now believe that its fabrication is overly complex particularly because of the curved grid, which should be large and ellipsoidally shaped. Further, the preferred embodiments include an entrance section arranged along an axis generally perpendicularly to the axis of the rest of the cylindrically shaped chamber so that the overall size and weight of the analyzer are increased, thereby decreasing the usefulness of the design for space applications. The reference also describes a coaxial design, but this design requires the electron source, typically a sample being irradiated by probe particles or radiation, to be inserted into the middle of the high-vacuum coaxial analyzer. Such a sample insertion is disadvantageous for remote high-throughput operation as required for a space application or even for an industrial application. In any case, a sample apparatus located in the beam path between the low-pass and high-pass filters is bound to absorb some of the desired back-reflected electrons and reduce the throughput of the analyzer.
Tepermeister et al. disclose a coaxial two-section analyzer in “Modeling and construction of a novel electron energy analyzer for rapid x-ray photoelectron spectroscopy spectra acquisition,” Review of Scientific Instrumentation, vol. 62, no. 8, August 1992, pp. 3828-3834. However, the Tepermeister design includes two large curved grids between its two sections and does not control the energy of the particles incident on the first section and does not focus them before entering the first section. Thus, the Tepermeister analyzer is considered to be large, difficult to build, and provide low throughput.
A compact, economical, and efficient charged particle analyzer is thus still needed for many applications both in the laboratory and commercial production line and in demanding space applications.